F# – Pipe Forward and Pipe Backward

You can become a serverless blackbelt. Enrol to my 4-week online workshop Production-Ready Serverless and gain hands-on experience building something from scratch using serverless technologies. At the end of the workshop, you should have a broader view of the challenges you will face as your serverless architecture matures and expands. You should also have a firm grasp on when serverless is a good fit for your system as well as common pitfalls you need to avoid. Sign up now and get 15% discount with the code yanprs15!

I’m taking a bit of time to brush up my knowledge of F# and see if I can write better F# code and one of the things I notice is that whilst I use pipe-forward operator (|>) often when working with collections I don’t nearly use the pipe-backward operator (<|) as frequently as I should. It makes sense as the pipe-forward operator works similar to the way LINQ and IEnumerable works, just to remind myself and others like me how these work:

Pipe-forward operator (|>)

Pipe-forward operator lets you pass an intermediate result onto the next function, it’s defined as:

let (|>) x f = f x

For instance, to apply a filter (i.e. IEnumerable.Where) for even numbers to a list of integers from 1 to 10, you can write:


You can add further processing steps to this intermediate result:


Forward composition operator (>>)

The forward composition operator lets you ‘compose’ functions together in a way similar to the way the pipe-forward operator lets you chain function delegates together, it is defined as:

let (>>) f g x = g (f x)

Now imagine if you have two functions:


You can use them to build a high-order function that returns triples the square of a float, n, using the >> operator:


This is syntactically cleaner and easier to read than:


and it’s especially useful when chaining together a large number of functions.

Pipe-backward operator (<|)

The pipe-backward operator takes a function on the left and applies it to a value on the right:

let (<|) f x = f x

As unnecessary as it seems, the pipe-backward operator has an important purpose in allowing you to change operator precedence without those dreaded parentheses everywhere and improve readability of your code,.

For example:


can be written as


Backward composition operator (<<)

The inverse of the forward composition operator, the << operator takes two functions and applies the right function first and then the left, it’s defined as:

let (<<) f g x = f (g x)

Mostly I find it more suitable than the forward composition operator in cases where you want to negate the result of some function, for example, to find the odd numbers in a list:


Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my weekly newsletter

Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.

Skill up your serverless game with this hands-on workshop.

My 4-week Production-Ready Serverless online workshop is back!

This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.

We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

If you enrol now you can also get 15% OFF with the promo code “yanprs15”.

Enrol now and SAVE 15%.

Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!

Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!